The demand for remote access to information sources and data retrieval, as evidenced by the success of services such as the World Wide Web, is a driving force for high-speed network access technologies. The public switched telephone network (PSTN) offers standard voice services over a 4 kHz bandwidth. Traditional analog modem standards generally assume that both ends of a modem communication session have an analog connection to the PSTN. Because voice or modem signals are typically converted from analog to digital when transmitted towards the PSTN and then from digital to analog when received from the PSTN, data rates may be limited to 33.6 kbps as defined in the ITU-T Recommendation V.34 developed by the International Telecommunications Union (ITU).
The need for an analog modem may be eliminated, however, by using the basic rate interface (BRI) of the Integrated Services Digital Network (ISDN). A BRI offers end-to-end digital connectivity at an aggregate data rate of 160 kbps, which is comprised of two 64 kbps B channels, a 16 kbps D channel, and a separate maintenance channel. ISDN offers comfortable data rates for Internet access, telecommuting, remote education services, and some forms of video conferencing. ISDN deployment, however, has generally been very slow due to the substantial investment required of network providers for new equipment. Because ISDN is not very pervasive in the PSTN, the network providers have typically tariffed ISDN services at relatively high rates, which may be ultimately passed on to the ISDN subscribers. In addition to the high service costs, subscribers must generally purchase or lease network termination equipment to access the ISDN.
While most subscribers do not enjoy end-to-end digital connectivity through the PSTN, the PSTN is nevertheless mostly digital. Typically, the only analog portion of the PSTN is the phone line or local loop that connects a subscriber or client modem (e.g., an individual subscriber in a home, office, or hotel) to the telephone company's central office (CO). Local telephone companies have been replacing portions of their original analog networks with digital switching equipment. Nevertheless, the connection between the home and the CO has been the slowest to change to digital as discussed in the foregoing with respect to ISDN BRI service. A recent data transmission recommendation issued by the ITU, known as ITU-T V.90, takes advantage of the digital conversions that have been made in the PSTN. By viewing the PSTN as a digital network, V.90 technology can accelerate data downstream from the Internet or other information source to a subscriber's computer at data rates of up to 56 kbps, even when the subscriber is connected to the PSTN via an analog local loop.
To understand how the V.90 Recommendation achieves this higher data rate, it may be helpful to briefly review the operation of V.34 analog modems. V.34 modems are generally optimized for a configuration in which both ends of a communication session are connected to the PSTN by analog lines. Even though most of the PSTN is digital, V.34 modems treat the network as if it were entirely analog. Moreover, the V.34 Recommendation assumes that both ends of the communication session suffer impairment due to quantization noise introduced by analog-to-digital converters. That is, the analog signals transmitted from the V.34 modems are sampled at 8000 times per second by a codec upon reaching the PSTN with each sample being quantized to a discrete PCM quantization level that is represented by an eight-bit pulse code modulation (PCM) codeword. The codec uses 256, non-uniformly spaced, PCM quantization levels defined according to either the μ-law or A-law coding law (i.e., the ITU G.711 Recommendation).
Because the analog waveforms are continuous and the binary PCM codewords are discrete, the digits that are sent across the PSTN can only approximate the original analog waveform. The difference between the original analog waveform and the reconstructed quantized waveform is called quantization noise, which limits the modem data rate.
In a V.34 connection, quantization noise may limit data rates to less than 33.6 kbps. The V.90 standard relies on the lack of 8-bit analog-to-digital conversions in the downstream path, to enable downstream data transmission at up to 56 kbps.
The general environment for which the V.90 standard has been developed is depicted in FIG. 1. An Internet Service Provider (ISP) 22 is connected to a subscriber's computer 24 via a V.90 digital server modem 26, through the PSTN 28 via digital trunks (e.g., T1, E1, or ISDN Primary Rate Interface (PRI) connections), through a central office switch 32, and finally through an analog loop to the client's modem 34. The central office switch 32 is drawn outside of the PSTN 28 to better illustrate the connection of the subscriber's computer 24 and modem 34 into the PSTN 28. It should be understood that the central office 32 is, in fact, a part of the PSTN 28. The operation of a communication session between the subscriber 24 and an ISP 22 is best described with reference to the more detailed block diagram of FIG. 2.
Transmission from the server modem 26 to the client modem 34 uses pulse amplitude modulation (PAM) and will be described first. The information to be transmitted is first encoded using only the 256 PCM codewords used by the digital switching and transmission equipment in the PSTN 28. These PCM codewords are transmitted towards the PSTN 28 by the PAM transmitter 36 where they are received by a network codec. The PCM data is then transmitted through the PSTN 28 until reaching the central office 32 to which the client modem 34 is connected. The PCM data are converted to analog voltage levels by the codec expander (digital-to-analog converter) 38 according to the PCM law. These voltage levels are processed by a central office hybrid 42 where the unidirectional signal received from the codec expander 38 is transmitted towards the client modem 34 as part of a bidirectional signal. A second hybrid 44 at the subscriber's analog telephone connection converts the bidirectional signal back into a pair of unidirectional signals. Finally, the analog signal from the hybrid 44 is converted into digital PAM samples by an analog-to-digital (A/D) converter 46, which are received and decoded by the PAM receiver 48. Note that for transmission to succeed effectively at 56 kbps, there must not be any 8-bit PCM analog-to-digital conversion between the server modem 26 and the client modem 34. Recall that analog-to-digital conversions in the PSTN 28 may introduce quantization noise, which may limit the data rate as discussed hereinbefore. The A/D converter 46 at the client modem 34, however, typically has a higher resolution than the A/D converters used in the analog portion of the PSTN 28 (e.g., 16 bits versus 8 bits), which results in less quantization noise. Moreover, the PAM receiver 48 needs to be in synchronization with the 8 kHz network clock to properly decode the digital PAM samples.
Transmission from the client modem 34 to the server modem 26 uses quadrature amplitude modulation (QAM) and follows the V.34 data transmission standard. That is, the client modem 34 includes a QAM transmitter 52 and a D/A converter 54 that encode and modulate the digital data. The hybrid 44 converts the unidirectional signal from the digital-to-analog converter 54 into a bidirectional signal that is transmitted to the central office 32. Once the signal is received at the central office 32, the central office hybrid 42 converts the bidirectional signal into a unidirectional signal that is provided to the central office codec. This unidirectional, analog signal is converted into either μ-law or A-law PCM codewords by the codec compressor (AID converter) 56, which are then transmitted through the PSTN 28 until reaching the server modem 26. The server modem 26 includes a conventional V.34 receiver 58 for demodulating and decoding the data sent by the QAM transmitter 52 in the client modem 34. Thus, data is transferred from the client modem 34 to the server modem 26 at data rates of up to 33.6 kbps as provided for in the V.34 standard.
As described above, the digital portion of the PSTN 28 transmits information using eight-bit PCM codewords at a frequency of 8000 Hz. Thus, it would appear that downstream transmission should take place at 64 kbps rather than 56 kbps as defined by the V.90 standard. While 64 kbps is a theoretical maximum, several factors prevent actual transmission rates from reaching this ideal rate. First, even though the problem of quantization error has been substantially reduced by using PCM encoding and PAM for transmission, additional noise in the network or at the subscriber premises, such as non-linear distortion and crosstalk, may limit the maximum data rate. Furthermore, the PCM encoding technique does not use uniformly spaced voltage levels for representing data. The PCM codewords representing very low levels of sound have PAM voltage levels spaced close together. Noisy transmission facilities may prevent these PAM voltage levels from being distinguished from one another thereby causing loss of data. Accordingly, to provide greater separation between the PAM voltages used for transmission, not all of the 256 PCM codewords are used.
It is generally known that, assuming a convolutional coding scheme, such as trellis coding, is not used, the number of symbols required to transmit a certain data rate is given by Equation 1:bps=R5log2N5  EQ. 1where bps is the data rate in bits per second, Rs, is the symbol rate, and N5 is the number of symbols in the signaling alphabet or constellation. To transmit at 56 kbps using a symbol rate of 8000, Equation 1 can be rewritten to solve for the number of symbols required as set forth below in Equation 2:N5=256000/8000=128  EQ.2Thus, the 128 most robust codewords of the 256 available PCM codewords are chosen for transmission as part of the V.90 standard.
The V.90 Recommendation, therefore, provides a framework for transmitting data at rates up to 56 kbps provided the connection is capable of supporting the PCM transmission mode. The most notable requirement is that there can be at most one digital-to-analog conversion and no 8-bit PCM analog-to-digital conversion in the downstream path in the network. Nevertheless, other digital impairments, such as robbed bit signaling (RBS), digital mapping through attenuation pads (PAD), which results in attenuated signals, and code conversion between different PCM coding laws may also inhibit transmission at V.90 rates.
The V.90 Recommendation provides for the use of a digital impairment learning (DIL) signal, which is transmitted from the server/digital modem to the client/analog modem during phase three of a multi-phase startup procedure. The client/analog modem receives and analyzes the DIL signal to determine accurate estimates of the signal levels corresponding to transmitted PCM codes as well as to identify digital impairments. Moreover, the DIL signal is a programmable signal with the signal content being specified through use of a DIL descriptor. The DIL descriptor includes several fields that comprise a compact representation of the complete DIL signal. The client/analog modem fills in these fields and transmits the DIL descriptor to the digital/server modem, where the DIL signal is generated based on the specifications contained in the DIL descriptor. The downstream communication channel from the μ-law or A-law expander 38 to the A/D converter 46 in the analog/client modem 34 is typically subject to analog impairments, such as frequency-dependent attenuation, non-linear distortion, and white or colored noise sources that may further distort the D/T, signal. For example, transformers typically exist wherever a 2-wire to 4-wire or 4-wire to 2-wire transformation occurs, and they may introduce nulls at direct current (DC) and severe non-linear distortion below 100 Hz. Line cards in the central office typically include a low-pass interpolation filter, which has a sharp roll-off around 4 kHz (i.e., the Nyquist frequency for the PSTN).
Consequently, there exists a need for DIL signals having improved performance in the presence of both digital and analog impairments and systems and methods for generating same.